Fifth generation (5G) wireless telecommunications systems are currently under development. One aspect of this development is the specification by 3GPP of an NR interface intended to provide new and/or improved capabilities compared to prior radio interfaces. Systems employing such an interface, commonly referred to as NR systems, may provide, e.g., greater traffic capacity, lower latency or higher data rates.
Mobile broadband is expected to continue as a main driver of the demand for high overall traffic capacity and high achievable end-user data rates. Several use-cases and deployment scenarios will require data rates of up to 10 Gbps, for example. These demands can be addressed by networks with distances between access nodes ranging from a few meters in indoor deployments to roughly 50 m in outdoor deployments, i.e., with an infrastructure density considerably higher than most dense networks of today. The wide transmission bandwidths needed to provide data rates of up to 10 Gbps and above can likely be obtained from spectrum allocations in the centimeter and millimeter-wave bands. High-gain beamforming, typically realized with array antennas, can be used to mitigate the increased pathloss at higher frequencies, and benefit from spatial reuse and multi-user schemes. Accordingly, these and other features are expected to be adopted as part of NR systems.
Besides using traditional licensed spectrum bands, NR systems are expected to operate in unlicensed bands and license-shared bands, especially for enterprise deployment scenarios. Consequently, coexistence support is needed to enable efficient spectrum sharing among different operators and/or other systems. One way to achieve this coexistence is through a Listen-before-talk (LBT) mechanism, which is a distributed mechanism that avoids a need to exchange information between different coexisting systems. While LBT has been effective in providing spectral coexistence for wide beamwidth transmissions, numerous studies (See e.g., FIG. 3) have shown that LBT is somewhat unreliable for highly directional transmissions.
In typical deployments of wireless local access networks (WLAN), carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed for a clear channel assessment (CCA), and a transmission is initiated only if the channel is deemed to be idle. If the channel is deemed to be busy, the transmission is deferred until the channel is deemed to be idle. When the range of several access points (APs) using the same frequency overlap, transmissions related to one AP may be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they must share the channel in time, and the throughput for the individual APs may be severely degraded compared to their isolated deployments. A general illustration of the LBT mechanism is shown in FIG. 1.
After a Wi-Fi station “A” transmits a data frame to a station “B”, station B shall transmit an acknowledgement (ACK) frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing the LBT operation. To prevent another station from interfering with such an ACK frame transmission, a station must defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before a subsequent attempt to assess again whether or not the channel is occupied.
Therefore, a station that intends to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is found to be idle, the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random back off period.
To further prevent a station from occupying the channel continuously and thereby preventing other stations from accessing the channel, a station intending to transmit again after a transmission is completed must perform a random back off.
The PIFS is used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it can be used by stations operating under point coordination function (PCF), to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the station shall sense the medium. When a station determines that the medium is idle for one PIFS period (generally 25 μs), the station shall transmit a Beacon frame containing a coordination function (CF) Parameter Set element and a delivery traffic indication message element.
The widely used Wi-Fi systems based on IEEE 802.11b/g/n/ac standards operate in sub 6 GHz frequencies (2.4 and 5 GHz frequencies), and listen and talk operations, i.e., sensing, reception and transmission are predominantly omni-directional. An objective of LBT is to avoid interference between simultaneous data transmission. Practical application results show that this works well in this case.
In License Assisted Access (LAA) systems, an eNB may transmit information on a physical downlink shared channel (PDSCH) of an LAA Scell after first sensing the channel medium to be idle during the slot durations of a defer duration Td; and after the counter N is zero in step (4). The counter N is adjusted by sensing the channel for additional slot duration(s) according to steps (1)-(6) below:                (1) Set N=Ninit, where Ninit is a random number uniformly distributed between 0 and CWp, and go to step (4);        (2) If N>0 and the eNB chooses to decrement the counter, set N=N−1;        (3) Sense the channel for an additional slot duration, and if the additional slot duration is idle, go to step (4); else, go to step (5);        (4) If N=0, stop; else, go to step (2).        (5) Sense the channel during the slot durations of an additional defer duration Td;        (6) If the channel is sensed to be idle during the slot durations of the additional defer duration Td, go to step (2); else, go to step (5);        
If an eNB has not finished its transmission on PDSCH after step (4), the eNB may continue transmission after sensing the channel to be idle at least in slot durations of an additional defer duration Td.
The defer duration Td includes a duration 16 μs≤Tf≤, 16 μs+Ts immediately followed by mp consecutive slot durations, where each slot duration is 9 μs≤Tsl≤9 μs+Ts, and Tf includes an idle slot duration Tsl at start of Tf.
A slot duration Tsl is considered to be idle if the eNB senses the channel during the slot duration, and the power detected by the eNB for at least 4 μs within the slot duration is less than an energy detection threshold XThresh. Otherwise, the slot duration Tsl is considered to be busy.
CWmin, p≤CWp≤CWmax, p is a contention window.
CWmin, p and CWmax, p are chosen before step (1) of the above procedure.
mp, CWmin, p and CWmax, p are based on channel access priority class associated with the eNB transmission, as shown in Table 1 below.
If the eNB transmits discovery signal transmission(s) not including PDSCH when N>0 in the procedure above, the eNB shall not decrement N during the slot duration(s) overlapping with discovery signal transmission.
The eNB shall not continuously transmit on a channel on which the LAA Scell(s) transmission(s) are performed, for a period exceeding Tm cot, p, as shown in Table 1 below.
For p=3 and p=4, if the absence of any other technology sharing the carrier can be guaranteed on a long term basis (e.g. by level of regulation), Tm cot, p=10 ms, otherwise Tm cot, p=8 ms.
TABLE 1Channel Access Priority ClassChannel AccessPriority Class (p)mpCWmin,pCWmax,pTm cot,pallowed CWp sizes1137 2 ms{3,7}21715 3 ms{7,15}33156338 or {15,31,63}10 ms471510238 or {15,31,63,127,10 ms255,511,1023}
As indicated above, LBT is somewhat unreliable for highly directional transmissions. Consequently, there is a general need for improved or alternative approaches for coexistence in unlicensed or license-shared spectrum bands.